In a step-down DC-to-DC converter, a synchronous rectification method and a chopper method are used. The synchronous rectification method is a method in which an inductor is driven by alternately turning on or off a high-side switch and a low-side switch. The chopper method is a method in which only the high-side switch is turned on or off.
The chopper method has less switching loss at a small current, but since a low-side element is formed of a diode, a voltage drop by the diode is large, and efficiency at a large current is low. On the other hand, in the synchronous rectification method, since switching is made on the low-side, too, the efficiency at the large current is high, but switching loss is large at the small current, and efficiency is deteriorated. Also, a current of the inductor flows in the opposite direction through the low-side switch at the small current, which might deteriorate efficiency.